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Selective Mortality With
Prescribed Fire In Canyon
Live Oak 1
Timothy E. Paysen and Marcia G. Narog 2
~·
'•
·./
Canyon live oak (Quercus chrysolepis Liebm.) is a common hardwood
species in southern California. In recent years it has become necessary to
actively manage this non-timber type
because of its increased use by the
public for fuelwood and recreation
(Thornburgh, in press). Little information exists to guide management
decisions for improving the quality
of canyon live oak stands (Plumb and
McDonald 1981).
The influence of natural fire in the
evolution of many oak species
(Plumb and McDonald 1981, Rouse
1986) suggests that a prescribed
burning program would be feasible
and desirable. Some believe that prescribed burning in oaks could be
used to rejuvenate decadent or overgrown stands (Graves 1977, Hannah
1987).
Canyon live oak bark bums easily
(Plumb and Gomez 1983). This may
create problems during understory
burning if fire carries up the trunk.
However, many oak species are extremely resilient to severe stress or
injury. Total foliar loss due to
drought stress (McCreary 1988) or
heavy bole charring due to fire
'Poster paper presented at the conference, Effects of Fire in Management of
Southwestern Natural Resources (Tucson,
AZ, November 74-77, 1988).
2
Research Forester and Ecolo~]isf. Forest
Fire Laboratory, Pacific Southwest Forest
and Range Experiment Station, Forest Service, U.S. Department of Agriculture. Riverside, CA.
LEGEND
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1
Thin and Elur-n
Thin
[ill
Control
()
Ridge11 ne
Figure 1.-Study area at Skinner Ridge, with plot layout and location of thin and bum plots
18, 28, and 38.
(Plumb and Gomez 1983) do notalways cause tree mortality.
This paper will address the first
part of a larger study evaluating
thinning and prescribed burning in
canyon live oak to (1) improve stand
quality through selective thinning,
(2) reduce fire hazard through the
establishment of a shaded fuel break
and (3) increase diversity of wildlife
habitat. We will analyze data gathered on crown recovery and tree
mortality 32 months after the prescribed burn.
241
Methods
The study was conducted in a
closed-canopy canyon live oak forest,
on the north slope of Skinner Ridge
(elev. 1500 m), on the San Bernardino
National Forest in southern California. Aspect varied slightly among
plots (fig. 1), and slope ranged from
quite steep (35°) and convex to moderate and concave. Litter varied in
depth, but was relatively complete
over all plots. Wildfire burned the
site about 100 years ago.
The study area was divided into
three blocks, each containing three
plots which measured 30m by 40 m.
Within each block, three treatments,
control, thin, and thin and burn,
were randomly assigned to the plots.
The oaks were thinned from a stand
basal area of 54 m 2 /ha to 22.5 m 2 /ha.
Thinning was as equitable as possible
across diameter size classes, and between single stemmed trees and
stems from multiple-stemmed clusters. Stand thinning, completed in
June 1985, took 15 months. The prescribed burn treatment was accomplished during November 1985.
Three months after the bum, mortality, diameter, bole injury, and percent live crown were recorded for all
oaks with stems inside the perimeter
of each thin and burn plot. Four 3-mwide strips, which were horizontal to
slope, were positioned every tenth
meter in each plot (fig. 2), and were
used as subsamples for subsequent
data collections in May 1987 and August 1988.
We assessed above ground stem
mortality of the oaks based on crown
and bole condition. Root survival
was not considered a criterion for
tree survival, even though canyon
live oak commonly resprou t from the
root area.
Percent crown vitality was visually estimated for each tree, based on
live foliage observed on branches
and twigs. Complete crown mortality
was assigned if no green foliage was
apparent in the tree crown. The
crown was designated as 100% live if
no scorched foliage or foliage loss
was evident. Estimates were for all
percentages of live crown between
these two extremes.
Bole damage was evaluated by a
visual inspection of the bark and
cambium. A bark scraper was used
to assess damaged tissue. Obvious
color or texture deviations from
those observed in living oak tissue
were considered to be evidence of
fire injury. Classification of injury
was based on the percent circumference damaged by the fire.
Results
Flames did not reach the canopy
so consumption did not occur in the
tree crowns. But, heat convection
from the fire did cause extensive foliar injury in most trees. Initial assessment of injury showed that bole
and crown damage did not always
occur together or to the same degree.
Over 98% of 635 oaks on the thin and
burn plots sustained some injury
from the fire. Ninety-eight percent of
the trees had some degree of crown
injury, while obvious bole damage
was found in only 64% of the trees.
In January 1986, mean crown
damage per tree (limited to foliar
death) from the fire was 76%. Foliar
death of the crowns continued, and
by spring 1986 all trees or portions of
trees within the perimeter of the bum
appeared to be dead. In May 1987, a
subsample of the burned plots
showed 37% mean crown recovery
meters
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10
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10
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Figure 2.-Subsample layout for thin and
burn plots (1 8, 28, 38).
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.
20
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DIAMETER CLASS (em}
Figure 3.-Live and dead DIAMETER frequencies for all plots.
242
per tree as evidenced by refoliation.
By August 1988, overall recovery for
the three plots had reached 47%
(table 1).
After 32 months, overall tree mortality was 23%. Sixty-nine percent of
all bole injured trees showed some
heat injury on 50% to 100% of the
bole circumference, within a vertical
range of 4 m from the ground. With
only a few exceptions on plot 1B, the
oaks that died had smaller diameters
than those that survived (fig. 3). Plot
one was steeper and deeper layers of
slash had accumulated around tree
bases. The diameter of the live vs.
dead trees constituted two distinct
populations. According to the RankSum Test this difference was highly
significant (right-handed tail probability= 0.9999).
Conclusions
The preliminary results presented
in this paper suggest that prescribed
understory burning probably can be
used to manage canyon live oak. After 32 months, post burn mortality of
trees was low-23%, even though
fire injury was sustained by most
trees-98%. Greater mortality occurred in trees smaller than 20 em
diameter, indicating that understory
burning may prove valuable for selective thinning. Mortality of the few
larger diameter trees probably resulted from long residence times of
burning around the base of the trees.
In general, canyon live oak was able
to withstand understory burning despite bole and foliar injury.
Continued monitoring in this canyon live oak stand is necessary before a final decision can be made regarding improvement of stand quality after burning. Plumb and Gomez
(1983) found that oak crown mortality can occur as many as 8 years after
a fire. Post fire effects, yet to be analyzed, include possible changes in
tree growth rate or pattern, susceptibility to pests and diseases, and vigor
of remaining trees.
Acknowledgments
We would like to thank Wilmer
Stillens, David Van Dusen, and Steven Bunyak, U.S. Forest Fire Laboratory, U.S. Forest Service, for their assistance in the collection of field data.
References
Graves, Walter C. 1977. Oaks provide habitat for 86 species of birds,
11 mammals. Outdoor California.
September/October: 17-18,33.
Hannah, Peter R. 1987. Regeneration
methods for oaks. Northern Journal of Applied Forestry. 4: 97-101.
McCreary, Douglas. 1988. Brown
trees bounce back. Oaks 'N Folks.
Cooperative Extension, USDA
University of California, Berkeley,
CA. 3(1):1
Plumb, Timothy R.; Gomez, Anthony
P. 1983. Five southern California
oaks: Identification and postfire
management. USDA For. Serv.
Gen. Tech. Rep. PSW-71. 56 p.
Plumb, Timothy R.; McDonald,
Philip M. 1981. Oak management
in California. USDA For. Serv.
Gen. Tech. Rep. PSW-54. 11 p.
Rouse, Cary. 1986. Fire effects in
northeastern forests: oak. USDA
For. Serv. Gen. Tech. Rep. NC-105.
7p.
Thornburgh, Dale A. Quercus
chrysolepis Fagaceae, Beech Family
Quercus chrysolepis Liebm. Canyon
live oak, For. Dept., Humboldt
State Univ., Arcata, CA. In press.
243